A. Field of the Invention
This invention relates to RF switch assemblies and in particular to control of RF switch assemblies.
B. Background Art
U.S. Pat. No. 4,326,289, "Expandable Communications Systems", filed Feb. 28, 1980 disclosed a broadband RF communication system having a central station, a plurality of subordinate stations that can be selectively addressed by the central station and a bidirectional transmission medium, for example a coaxial cable. The cable linked the subordinate stations of the system to the central station where a central controller was capable of addressing a specific subordinate station and exchanging control and message information with any address station.
Because the typical subordinate station was physically located on the premises of a customer, the station may have been vulnerable to unauthorized tampering. The possibility of tampering could be deterred by locating the system stations beyond the customer's premises. For example, the stations may have been located on utility poles or within cable vaults. However, in large computerized plants where communication was desired between many areas within the plant, these stations had to be distributed at many points where they were prone to tampering.
Additionally, in large networks there may be many miles of cable which are capable of picking up ingress signals. Ingress signals can include such things as noise and spurious signals which tend to be gathered by the numerous nodes of the system. In the case of an industrial RF network, ingress can cause communications outages which are very difficult to resolve.
It is desirable to detect ingress as soon as possible in order to try to resolve it before it causes an outage because down time in an industrial system may be extremely expensive. However spotting ingress in the prior art required visually observing it on a spectrum analyzer or the headend status monitor. Constant operator monitoring of these devices is tedious and unreliable.
In order to resolve outages caused by ingress or to prevent outages when ingress is first detected it is necessary to isolate the source of ingress in the network. A conventional method of ingress isolation was to systematically disconnect feeder cables throughout the system while an assistant at the headend observed whether the interfering signal was affected. The disconnecting of feeder cables was required in previous systems because these systems did not include local switches for locking out sections of the network.
In practice, this process required extensive walking around the plant floor with a ladder in order to access the feeder cables which were located substantially above head level to avoid damage. Two-way radio communication between the person disconnecting the feeder cables and the person at the headend was required throughout the process. This approach was very cumbersome and disruptive.
It is also known in the prior art to locate ingress using status monitors distributed in the network. These status monitors included RF switches having three modes which routed RF signals straight through the switch, attenuated the signals as they passed through the switch, or blocked the signals from passing through the switch thereby locking the signal out in accordance with control signals sent from the headend. Thus, an operator could observe the ingress at the headend and remotely operate selected status monitors to determine whether activation of each selected status monitor affected the ingress observed at the headend. The conventional method for selecting status monitors for remote operation of the associated RF switches to locate the source of the ingress was a bisection method. In the bisection method the network was divided in half. One half of the network was attenuated or locked out and the effect upon the observed ingress was determined. If the ingress disappeared then the half of the network which was attenuated or locked out included the source of the ingress and this portion of the network was bisected. This process was continued until the source of the ingress was located.
However in the prior art, the status monitors, including the associated three-mode RF switches, were only used in connection with amplifiers which could only be distributed in the system at a limited number of points because of the high cost. Thus the number of points in the network tree which could be switched from the headend while observing the affect on ingress at the headend was limited.